Calcium phosphates are technologically significant in many areas. There is a wide range of calcium phosphate salts that exist at ambient conditions either as meta-stable or stable phases, the most common are listed in table 1 in order of increasing solubility(1) in aqueous solution. An amorphous calcium phosphate (ACP) phase has also been identified.
TABLE ICommon Calcium Phosphate SaltsChemical formulaCa/P RatioNameCa5(PO4)3(OH)1.67Hydroxyapatite (HAP)Ca4H(PO4)3•2.5H2O1.33Octacalcium phosphate (OCP)Ca3(PO4)21.50Tricalcium phosphate (TCP)CaHPO4•2 H2O1.00Dicalcium phosphate dihydrate(DCPD)Ca(H2PO4)20.50Monocalcium phosphate (MCP)Ca(H2PO4)2•H2O0.50Monohydrate calcium phosphate(MCPH)
Hydroxyapatite (HAP) is the most thermodynamically stable calcium phosphate salt at near ambient temperature and in the pH range 4 to 12. Significantly this includes physiological temperature and pH. HAP is used extensively in the catalysis, fertilizer and water treatment industries. It is also used in the biomedical and pharmacological arenas. Calcium phosphate-based biomaterials have been in use in medicine and dentistry for over 20 years because of their excellent biocompatibility with human tissues. Thus, hydroxyapatite has been widely used in dental implants, percutaneous devices, periodontal treatment, alveolar ridge augmentation, orthopedics, maxillofacial surgery, otolaryngology, and spinal surgery. The functionality of HAP as a biomaterial originates from its chemical and structural similarity to bone-apatite, the main inorganic component of the teeth and bones of invertebrates. HAP is a known osteoconductive material and is used both as a coating on metallic medical devices for hard-tissue arthroplasty and as a constituent in synthetic bone grafts and cements. HAP, being bio-mimetic, promotes bone in-growth and fixation invivo, a necessary attribute of any successful hard-tissue implant(2-3).
As a result of its intrinsic biocompatibility and the fact that many biologically species will readily adsorb on its surface, HAP is increasingly becoming the material of choice for the delivery of a wide range of therapeutic agents including antibiotics, growth factors and other regulatory and/or functional proteins involved in various genetic pathways of physiological significance). Habraken et. al. have written a comprehensive review of ceramic composite matrices as scaffolds for drug delivery, Calcium Phosphate and Calcium irected at eliminating the requirement for a sintering step in the manufacturing applications(5).
While extensively used in the hard tissue arena, the use of HAP as an alternative to polymers in drug delivery systems relevant to cardiovascular medical devices is currently under development for example MIV therapeutics VESTASYNC™ stent. Results recently reported on the clinical trial underway by MIV Therapeutics demonstrate no adverse clinical effects from the use of HAP-coated coronary stents(6).
In addition, hydroxyapatite has also been used as a biological chromatography support in protein purification and DNA isolation as in U.S. Pat. No. 4,798,886(7). Hydroxyapatite is also currently used for fractionation and purification of a wide variety of biological molecules, such as subclasses of enzymes, antibody fragments, and nucleic acids(8). Crystalline hydroxyapatite columns are commonly used in high-performance liquid chromatography. Typically, the chromatographic column is filled with irregularly shaped hydroxyapatite gels having poor mechanical strength.
It is known that spherical powders, in general, have better rheological properties than irregular powders and, thus, produce better coatings for hip implants and chromatographic separation. Spherical hydroxyapatite ceramic beads have recently been developed that exhibit improved mechanical properties and physical and chemical stability. However, these spherical ceramic beads are between 20-80 micron in size as in U.S. Pat. No. 5,858,318(9).
The electromechanical properties of HAP have recently attracted significant interest; in particular it has been shown that HAP is pyroelectric and possibly piezoelectric. Anecdotal evidence of the role of the electrical properties of HAP being of importance in physiological environment exists wherein polarised HAP has been shown to have improved bioactivity in simulated body fluid experiments as compared with its un-polarised counterpart. However to fully exploit the piezoelectric and or pyroelectric potential of HAP more controllable methods to manufacture single crystals and or anisotropic films or ceramic bodies of HAP are desirable.
Invitro, the response of osteoblasts (bone cells) to calcium phosphate particles or ceramics has been shown to be dependent on the topography and porosity of the materials at both the micro/meso and macro scales(10). Furthermore the porosity and surface chemistry of calcium phosphate particles and ceramics also affects their functionality as drug carriers, in particular the surface area available for adsorption and the chemical nature of the surface itself determines the amount of therapeutic that can be loaded and the subsequent elution profile achieved invivo.
The demand for synthetic well characterised HAP is driven by these high end purification and invivo applications. Consequently, much time and effort has been devoted to developing processes to manufacture Calcium Phosphate particles and ceramic bodies with tailored morphologies, topographies and porosities depending on the requirements of a specific application with the fundamental chemistry of HAP and Calcium Phosphates in general receiving much attention.
Several methods of preparing HAP and or depositing it onto surfaces have been reported including solid-state reaction pathways, plasma techniques, hydrothermal methods, layer hydrolysis of other calcium phosphate salts and sol-gel reaction methods among others(11-17).
The synthesis of HAP via hydrothermal routes by precipitation from supersaturated aqueous solutions is advantageous due to its low cost but all routes used to date have produced HAP crystals with poor crystallinity, often non-stoichiometric in composition. The difficulty with the production of high purity HAP crystals arises from the low solubility of Calcium phosphate phases in general in the pH regions of interest and as a consequence of the complicated nature of the phase diagram of the aqueous Ca2+/HPO42− system(1). Many reaction schemes have been proposed that use salts other than oxides or phosphates as the source of Calcium for HAP precipitation most common among them Calcium chloride and Calcium Nitrate. These salts are more soluble in aqueous solution giving highly supersaturated solutions with respect to [Ca] but as well as introducing contamination Cl or NO3 ions other possible contaminants are introduced with the PO4 source or to buffer the solution. Routes have been determined using carbonates, hydrogen phosphates, ammonium salts, potassium and sodium hydroxides, nitrates, urea, and chlorides all of which are possible contaminants capable of being introduced into the precipitating system with the reactants. Contamination of HAP with these ions gives rise to significant deviations in the crystallographic characteristics of the precipitated material. Furthermore the supersaturation (σ) conditions that prevail in such systems means that many precursor meta-stable phases are formed en route to HAP involving complex precipitation dissolution reactions mediated by surface chemistry phenomena. OCP, DCPD, TCP and ACP (amorphous Calcium phosphate) are the most commonly observed (kinetically stable) phases depending on temperature and pH. In many instances thermal treatment of the precipitated Calcium phosphate is often required resulting in non-stoichiometric HAP usually associated with a loss of hydroxyl ions at high temperature.
The most desirable hydrothermal route to HAP from a purity point of view would be the use of Ca(OH)2, CaO and H2O, and phosphoric acid mixed in the correct ratio as this would negate contamination of the final product with other ions. Such a reaction scheme has been proposed(18) but as the solubility of CaO is high relative to the solubility of HAP local super-saturations in the range of 10-20 exist in the early stages of the reaction, furthermore a number of meta-stable solid phase are formed en route to the final product. As a result of the slow kinetics of the transformations of these phases to HAP exceedingly long reaction times and intricate washing procedures must be applied during the process.
Importantly the supersaturation conditions that prevail in typical HAP synthesis routes where surface mediated secondary nucleation dominates, means that synthetic HAP generated by such methods is often amorphous or nanocrystalline. Furthermore dense bodies made by sintering such HAP powders are isotropic in nature.
Many disclosures are present that use spray drying as a means to HAP or other calcium phosphate salts but in all cases post processing sintering must be applied to yield the desired product.
The sol-gel method of HAP manufacture offers a molecular-level mixing of calcium and phosphorus precursors, which is capable of improving the chemical homogeneity of the resulting HAP to a significant extent(19).
Generally synthesis by the sol-gel process involves the mixing of Ca and P precursors, dissolved in an appropriate solvent, such as to yield a solution with the correct Ca to P molar ratio, 1.67. The resulting solution is typically aged to allow formation of a sol and or to remove excess solvent at which point it is sintered at high temperature to initiate reaction between the Ca and P species present. Many variations on this basic theme are reported in the literature employing different Ca and P precursor materials, solvents sintering temperatures and durations(20-32). The temperature that is required to form the apatitic structure depends largely on the chemical nature of the precursors but prolonged holding of the reactants at high temperature inevitably results in the degradation of the resulting HAP due to Hydroxyl loss.
The sol-gel process is also particularly versatile in that additional components can be incorporated to yield products with tailored composition. For example, substituted apatites can be manufactured by including appropriate amounts of the subsistent ions in the sol. Among the ions that are substituted into Calcium phosphates are silica, halogen ions, carbonate, magnesium, strontium, vanadium, arsenic, sulphate, alumina, zirconia and many others.
In addition the sol-gel process can be used to manufacture composite materials where the Calcium phosphate is present in conjunction with other phases. Materials have been made where Calcium Phosphate is present either as an adherent layer on a substrate or as a component in a composite body. Such compositions typically contain Titania, Silica, Alumina, Zirconia and other ceramics as separate phases(26, 27, 32-41).
While the sol-gel process has enjoyed a measure of success and offers a number of advantages, namely that it is cost effective and highly versatile, as a means to Calcium phosphate production its main draw back is the requirement of post reaction thermal processing. This inevitably gives rise to non-stoichiometric product (hydroxyl loss) and degradation of the desired product (HAP) to CaO and other Calcium phosphate salts. It is however clear from the literature that HAP precursor preparations that have a neutral or basic pH (often involving Ammonium Hydroxide) require higher sintering temperatures than those that are formed from acidic precursor sols such as the sol-gel preparations of Dean-Mo Liu et. al.(58). However a disadvantage with acidic precursors is the lack of hydroxyl species in the precursor solutions for incorporation into the HAP lattice.
Methods of manufacturing composite blasting particles in which HAP is a component have also been disclosed for blasting purposes. In this instance the HA is present within a glassy matrix with other harder materials which give the particle mass and density allowing impregnation of the HAP component on impact. Such particles are created using sol-gel techniques and again require prolonged sintering during there manufacture. A number of disclosures revolve around shot peening processes involving composite particles comprised of a dense core material and an outer adherent layer of softer material, for example the Rocatek junior bonding system™ for dental implants. In this instance composite particles comprising a dense core of Alumina and an outer adherent layer of silica are employed in the shot blasting of metallic implants the outer adherent layer of silica embedding in the surface on impact. The silica so embedded allows enhanced adhesion of further polymer layers attached to the implants through the use of silane coupling chemistry. Similar type particles have been disclosed wherein the outer layer is composed of Titania. The generation of such stratified particles comprising a dense core with outer adherent layers of Calcium Phosphate would have similar applications in the biomedical field but to date no flame pyrolysis method to manufacture them has been disclosed.
Flame spray pyrolysis and or oxidation have received much attention in recent years as a means to industrial scale production of many important inorganic compounds including nanoparticles. Examples include processes for the manufacture of Carbon black, Titania, Zinc oxide and fumed Silica among others. Many metallic powders and catalysts are also produced in Aerosol flame reactors. In such reactors solutions of appropriate precursor compounds are injected into a high temperature flame to initiate chemical reaction and yield the desired product(42-52). Flame spray reactors offer the advantage of being readily scalable to industrial production while simultaneously enabling a desired level of control over the morphology and size of the particles manufactured.
Such a scheme has recently been proposed for the manufacture of Calcium phosphate nano-particles including TCP and HAP(53). In this disclosure a solution of appropriate Ca and P precursors present as carboxylates in an aqueous solution are injected into a high temperature flame fuelled by methane and oxygen to initiate reaction. The resultant products however required a further thermal processing step (sintering) to yield the desired products. Cho et al.(54) have also recently reported a spray pyrolosis process for the manufacture of Hydroxyapatite nano particles involving the injection of an aqueous solution of Ammonium Phosphate and Calcium Nitrate into a high temperature flame fuelled by propane. The resulting particles however required a further sintering step to yield the desired material. Inoue et al.(55-57) teach combustion processes for the production of calcium phosphate materials utilising a mixture of aqueous and hydrocarbon solvent containing Ca and P precursors injected into a flame to initiate reaction. However to achieve complete dissolution of the precursors in the solvent mixture significant amounts of water are used in conjunction with the hydrocarbon. Even so an excess of acid must be used to prevent the precipitation of non-desirable Ca and P phases in the precursor solution prior to exposure to the flame. Given that residence times in the flame are very low the presence of significant volumes of water in the precursor solution is disadvantageous: latent heat must be supplied to evaporate this water reducing the temperatures in the flame and kinetically hindering the extent of reaction. As a result the product produced while having the correct Ca/P ratio is amorphous and must be sintered to yield crystalline material.